1. Field of the Invention
The present invention relates to an image forming apparatus of a cleanerless system, in which development means recovers a transfer residual toner, which remains on an image bearing member after a transfer step, thereby dispensing with a cleaner. More particularly, it relates to an image forming apparatus of cleanerless system, provided with an image bearing member (charged member onto which charges are applied) such as an electrophotographic photosensitive member or an electrostatic recording dielectric member, contact charging means (contact charging apparatus or direct charging apparatus) of a contact type, which has a charging member in contact with the image bearing member and in which a charging bias is applied to the charging member thereby charging the image bearing member, information writing means which forms an electrostatic latent image on a charged surface of the image bearing member, development means which develops the electrostatic latent image with a toner to form a toner image, transfer means which transfers the toner image on the surface of the image bearing member onto a recording medium, and auxiliary charging means which is in contact with the surface of the image bearing member and applies a bias thereto thereby erasing a hysteresis of a preceding image, wherein the development means recovers a transfer residual toner, which remains on an image bearing member after a transfer step, thereby dispensing with a cleaner.
2. Related Background Art
(a) Contact Charging Apparatus
In an image forming apparatus of an electrophotographic process or an electrostatic recording process, a corona charger has been commonly employed as charging means for charging an image bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric member at predetermined polarity and potential. Such corona charger is positioned in non-contact manner, in an opposed relationship to the image bearing member (hereinafter referred to as photosensitive member) and a surface thereof is exposed to a corona emitted from the corona charger thereby charging the surface of the photosensitive member to predetermined polarity and potential. Recently, there is being adopted a charging apparatus of contact type, in which a charging member (contact charging member) with a voltage application (charging bias) is maintained in contact with the photosensitive member to charge the surface of the photosensitive member at predetermined polarity and voltage because of advantages such as a lower ozone generation and a lower electric power consumption in comparison with the corona charger of non-contact type. In particular, an apparatus of roller charging type, utilizing a charging roller (conductive roller) as a charging member, is employed advantageously in consideration of stability of charging.
Also an apparatus of magnetic brush charging method in which a magnetic brush charging member (charging magnetic brush, hereinafter represented as a magnetic brush charger) having a magnetic brush portion formed by magnetically capturing magnetic particles as a contact charging member and such magnetic brush portion is contacted with the photosensitive member is also advantageously employed in consideration of the stability of the charging apparatus. In such magnetic brush charger, a magnetic brush is formed by magnetically capturing conductive magnetic particles either directly on a magnetic member or on a sleeve incorporating a magnetic member, and the magnetic brush is contacted in a stationary or rotating state with a photosensitive member and is given a voltage to charge the photosensitive member.
Also a member of conductive fibers formed into a brush shape (fur brush charging member or charging fur brush) or a conductive rubber blade (charging blade) formed by conductive rubber in a blade shape is also employed advantageously as a contact charging member.
The contact charging includes two charging mechanisms, namely a charging method principally based on a charge injection (charge injection charging system) and a charging method principally based on a discharge (contact charging system), in a mixture, and, the characteristics of these systems are exhibited depending on which system is governing. A charge-injection charging system executes charging of the surface of the photosensitive member by a direct charge injection from the contact charging member into the photosensitive member. More specifically, a contact charging member of a medium resistance of 107 to 1010 Ω·cm is contacted with the surface of the photosensitive member thereby executing a direct charge injection into the surface of the photosensitive member without principally relying on a discharge. Therefore, even in case a voltage applied to the contact charging member is lower than a discharge threshold value, the photosensitive member can be charged to a potential corresponding to the applied voltage. Such charge-injection charging system is not associated with an ozone generation by a discharge. However, in such charge-injection charging, the charging property is significantly influenced by the contact of the contact charging member with the photosensitive member. It is therefore required to form the contact charging member in a dense structure and to have a larger speed difference to the photosensitive member thereby obtaining a higher frequency of contact with the photosensitive member, and, in this respect, the magnetic brush charger can achieve a stable charging as a contact charging member.
The contact-charging discharge system charges the surface of the photosensitive member by discharge products resulting from a discharge phenomenon, generated in a small gap between a conduct charging member and the photosensitive member. In the corona charging, a voltage higher than the potential to be charged has to be applied to the contact charging member because a certain discharge threshold exists between the contact charging member and the photosensitive member, but this charging method shows discharge products significantly less than those in the corona charger and is simpler in configuration in comparison with the magnetic brush charger, thereby being advantageously employed.
(b) Cleanerless Process (Toner Recycling Process)
Image forming apparatuses have recently shown progresses in a compacter configuration, but the compactization of the entire image forming apparatus has a limitation by mere size reduction of means or devices of the image forming processes such as charging, exposure, development, transfer, fixation and cleaning. In the prior image forming process, a transfer residual toner remaining on the photosensitive member after the transfer step is collected as a waste toner by a cleaner (cleaning means) including a cleaning blade and a recovery container, but such waste toner is preferably absent also in consideration of the environmental protection.
Therefore, there is also realized an image forming apparatus of “cleanerless process” in which such exclusive cleaner is dispensed with and the residual toner on the photosensitive member is recovered by “recovery simultaneous with development” by development means and is re-used therein. The toner recovery simultaneous with development means a method of recovering the residual toner, remaining in a small amount on the photosensitive member after the transfer step, by a defogging bias (defogging potential difference Vback between a DC voltage applied to the development means and a surface potential of the photosensitive member) at the development in a next or subsequent cycle.
This method, in which the transfer residual toner is recovered by the development means and is used in a next or subsequent cycle, can eliminate the waste toner and can alleviate the works that have been involved in the maintenance of the cleaner. Also the absence of the cleaner provides a significant advantage in the space and allows to significantly reduce the dimension of the image forming apparatus.
Also in case of a charging apparatus of contact charging type, the transfer residual toner having a charge amount not recoverable by the developing apparatus (such toner being hereinafter called reversal toner) is once recovered by the charging member maintained in contact with the photosensitive member, then is shifted to a charge of a polarity recoverable in the developing apparatus, again released onto the photosensitive member and recovered by the developing apparatus.
(c) Image Hysteresis Erasing Member
A transfer residual toner, remaining on the surface of the photosensitive member after the transfer of the toner image and present in a pattern of a previous image, if passed through the charging apparatus in this state, results in a decrease of the charged potential in a portion of the previous or an interception of the exposure for next image formation, and thus affects a next developing process in the form of such image thereby resulting in a phenomenon that the previous image appears denser or paler in a next image (such phenomenon hereinafter called a ghost phenomenon).
Therefore, Japanese Patent Application Laid-open Nos. 2001-92330 and 2002-196620 propose an image forming apparatus equipped with an auxiliary charging member maintained in contact with the surface of the photosensitive member and applying a bias thereto, as an image hysteresis erasing member for erasing a residual hysteresis of the previous image. As such auxiliary charging member, a brush of conductive rayon fibers of a fiber length of 6 mm was contacted with the photosensitive member in a position between the transfer charger and the charging apparatus, and the brush was given a DC voltage of a positive polarity, opposite to the charging polarity of the toner and of the photosensitive member. This brush, when given a positive bias, effectively erases the hysteresis of the charged potential of the previous image, and the transfer residual toner is also perturbed by this brush. Such toner, upon being accumulated on the brush and reaching a limit amount, is returned in succession onto the photosensitive member. Thus, as the hysteresis of the previous image is already lost at the contact portion of the charging apparatus and the photosensitive member, the direct cause of ghost formation is eliminated.
However, in case the positive voltage applied to the aforementioned brush of conductive rayon (hereinafter called auxiliary charging brush) is excessively high, a contrast between the voltage applied to the auxiliary charging brush and a high potential portion on the photosensitive member after the transfer becomes excessively large to generate a discharge between the auxiliary charging brush and the photosensitive member, whereby, as a result, the transfer residual toner is charged by the auxiliary charging brush in a polarity opposite to the charging polarity at the development. In case the reversal toner generated by the auxiliary charging brush increases in amount, a large amount of the reversal toner is deposited on the charging member when it passes between the charging member and the photosensitive member.
The reversal toner deposited on the charging member is given a charge of a polarity same as that the of potential of the photosensitive member for example by a friction with the surface of the charging member at the charging nip or by a friction with the cleaning member of the charging apparatus, and is released onto the photosensitive member for avoiding the contamination of the charging member, but an increased deposited amount, resulting from an excessive generation of the reversal toner, exceeds the release amount from the charging member.
Consequently the toner is accumulated on the charging member, thereby leading to a defective charging of the charging member by a toner contamination, resulting in a fog in a developing area.
On the other hand, in case the voltage applied to the auxiliary charging brush becomes low, a contrast between the potential on the photosensitive member after the transfer and the voltage applied to the auxiliary charging brush is reduced, whereby the hysteresis of the previous image is not erased sufficiently and gives rise to a ghost phenomenon.
Therefore, the contrast between the voltage applied to the auxiliary charging brush and the potential on the photosensitive member after the transfer has to set at an appropriate value capable of avoiding the aforementioned drawback.
However, the potential of the photosensitive member after the transfer varies significantly depending on the image. For example, in an image with the image whose numbers of dots per unit area is high, the potential of the photosensitive member is about −300 V in average before the transfer, and becomes about −80 V after the transfer. On the other hand, in an image with the image whose numbers of dots per unit area is high, the potential of the photosensitive member is about −500 V in average before the transfer, and becomes about −250 V after the transfer. Therefore, in case the voltage applied to the auxiliary charging brush is constant regardless of the potential of the photosensitive member after the transfer, the aforementioned drawback may be experienced depending on the potential of the photosensitive member after the transfer. Particularly in case of image formations over a prolonged image, the electrostatic capacitance of the photosensitive member gradually increases to facilitate the retention of the potential of the previous image on the surface of the photosensitive member after the transfer, whereby an application of a constant brush bias expands the potential difference between the surface of the photosensitive member after the transfer and the auxiliary charging brush, and the toner tends to be charged in the polarity of the voltage applied to the auxiliary charging brush, depending on the potential of the photosensitive member after the transfer.